code: def test_expression_function_expr():
      # test including a expr.<CONSTANT> should return an ExprRef
      er = expr(expr.PI * 2)
      assert isinstance(er, ExprRef)
      assert repr(er) == "ExprRef({\n  expr: (PI * 2)\n})"
  
file_path: altair/tests/expr/test_expr.py
imports: import operator
  import pytest
  from altair import expr
  from altair import datum
  from altair import ExprRef
  from jsonschema.exceptions import ValidationError
label_desc: Use image node_id 11 for calling a global function with example usage: test_expression_function_expr() without return types